Giamatti will play Harold Levinson, the free-spirited playboy brother of Cora (Elizabeth McGovern) and uncle to Mary (Michelle Dockery) and Edith (Laura Carmichael). He'll be introduced in the Season 4 finale.

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Giamatti's past TV credits include the titular role in HBO's miniseries John Adams, which earned him an Emmy and a Golden Globe.

Downtown Abbey Season 4 premieres in the United States on Jan. 5 on PBS.